Organic solar cells (OSCs) have gained much attention in the past decade because they offer realization of low-cost solar energy conversion devices, with advantages including mechanical flexibility, light weight, ease of processing and large area roll-to-roll production. Most of the developments that have improved performance of OSCs are based on electron donor-acceptor heterojunctions, which deal with the photo-induced electron transfer from a donor conjugated polymer to an acceptor molecule. A revolutionary development in organic photovoltaics came in the mid-1990s with the introduction of a dispersed/bulk heterojunction (BHJ), where an electron accepting and an electron donating material are blended together at the length scale similar to the exciton diffusion length. The dissociation of excitons at the donor-acceptor interface leads to free electrons and holes which travel to the contacts, if continuous pathways exist in each material from the interface to the respective electrodes. The electron acceptors are often the fullerenes or the derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) having better miscibility in organic solvents. For OSCs with poly(3-hexylthiophene) (P3HT) as electron donor, highest efficiencies reaching up to 5% have been reported; however further improvement in efficiency is required for large scale commercialization and practical applications.
Among many factors, the crucial parameters that need to be taken into account for achieving high efficiency solar cells are the band gap for increased absorption, the photon-electron conversion rate and charge carrier mobility of the photoactive polymer. The high energy band gap of the polymer materials and weak absorbance poses a serious limitation on the capability to harvest sunlight. Moreover the charge carrier mobility of the materials is also moderate, which makes it necessary to keep the thickness of the active layer low to minimize recombination losses. Typical thicknesses of active layer in optimized BHJ devices is 100-200 nanometers (nm) due to the performance-limiting trade-off between optical absorption and electrical transport, which are adequate for visible light absorption owing to strong extinction coefficient in this wavelength range.
A significant portion of the solar flux lies beyond λ=600 nm, which is near the band edge of many BHJ materials. Recently many low-band gap polymers have been produced with broader absorption range to better harvest the solar spectrum, exhibiting efficiencies of over 5%. A few reports on these low-band gap polymers demonstrated internal quantum efficiency, IQE (fraction of collected carriers per absorbed photon) approaching 100%. However, these low-band gap polymers suffer from poor absorbance (fractional number of absorbed photons from the solar spectrum) in their absorption range, for thicknesses optimized for electrical performance. Hence absorbance of these low-band gap polymer based OSCs has to be improved, since it also governs the photocurrent generation, like IQE, and seeks attention to further push the power conversion efficiencies towards the desired 10% mark.
The constraints on active layer thickness of these OSCs make it imperative to develop light trapping schemes to enhance absorption in a specific spectral range with weak absorption without increasing photoactive layer thickness. Light management techniques in ray optics regimes, have been implemented for enhancement in optical absorption, e.g. collector mirrors, microprism substrates and V-folded configurations. Light trapping by means of a periodic patterning, has been achieved either by using a corrugated substrate: buried nanoelectrodes, microprism substrates and azo-polymer-based sub-micrometer periodic surface structures, or by patterning the active layer into photonic crystal structures or surface relief gratings at the optical length scale, using soft lithographic techniques such as soft-embossing and PRINT.
Photoactive layer texturing methods based on soft lithography, have shown greater promise, but in these schemes absorption enhancement was not tailored to a desired spectral range. Furthermore, these techniques require additional processing of the active layer, which increases the risk of contamination and oxidation of the active layer surface, leading to Schottky barrier formation at the metal/polymer interface.
Moderate success has been achieved for patterned substrates targeting enhanced light absorption, with small molecules which can be thermally evaporated and conformal coating is achieved, irrespective of the underlying texture. However, challenges in obtaining a conformal coverage of solution processed polymers on these textured substrates, with coating technologies such as spin-coating, spray coating, inkjet printing, doctor blading, gravure and flexographic printing, remains a serious limitation of these periodic structures. Attempts at spin coating a BHJ layer onto textured substrates have led to over filling of the valleys and shunts at the crest, which can severely restrain the electrical performance of these OSCs, owing to high charge-carrier recombination and leakage current.
Embodiments of the present invention overcomes such problems and provides a structure and technique for conformal coating of polymer photovoltaic layers on light trapping textured substrates. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.